[blau911]

The short answer is the following:
1) Rust concerns
2) Maintenance concerns
3) Import costs
The first is self explanatory. As for the second, because Canada is a very large country with relatively few people (compared to the USA), qualified Porsche specialised mechanics are few and far between. hence the concern about the quality of maintenance and repair work. Finding a Porsche mechanic in Toronto? No problem. How about Saskatchewan? Exactly...
As for import costs, Canada requires that the importer use a state registered import agent when buying a car from canada and bringing it into the USA. Last I checked, they aren't cheap. Then there is the USA customs tax.
Hope that helps...